docs: add instructions of starting playground with cargo run

[fuyufjh]

What's changed and what's your intention?

As title

Checklist

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Refer to a related PR or issue link (optional)

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[codecov]

Codecov Report

Merging #1952 (2903487) into main (abd2f98) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1952      +/-   ##
==========================================
+ Coverage   70.87%   70.91%   +0.04%     
==========================================
  Files         618      618              
  Lines       80087    79964     -123     
==========================================
- Hits        56763    56709      -54     
+ Misses      23324    23255      -69     

Flag	Coverage Δ
rust	70.91% <ø> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/connector/src/base.rs	0.00% <0.00%> (ø)
src/connector/src/kafka/split.rs	0.00% <0.00%> (ø)
src/connector/src/pulsar/split.rs	0.00% <0.00%> (ø)
src/connector/src/kinesis/split.rs	0.00% <0.00%> (ø)
src/connector/src/kafka/source/message.rs	0.00% <0.00%> (ø)
src/connector/src/pulsar/source/message.rs	0.00% <0.00%> (ø)
src/connector/src/kinesis/source/message.rs	0.00% <0.00%> (ø)
...nnector/src/filesystem/s3/source/s3_file_reader.rs	0.00% <0.00%> (ø)
src/frontend/src/expr/type_inference.rs	97.37% <0.00%> (+1.22%)	⬆️
src/storage/src/hummock/local_version_manager.rs	86.48% <0.00%> (+4.72%)	⬆️
... and 2 more

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